This application is based on and incorporates herein by reference Japanese Patent Applications No. 2001-107101 filed on Apr. 5, 2001 and No. 2002-14741 filed on Jan. 23, 2002.
The present invention relates to an ion current detecting device for an internal combustion engine.
Combustion conditions in an internal combustion engine continuously vary depending on driving conditions of a vehicle. To maintain good combustion conditions, abnormal combustion conditions, such as a misfire, are detected by measuring an ion current which is generated during combustion. Based on results of the abnormal combustion detection, ignition timing of spark plug and air-fuel ratio of air-fuel mixture are controlled. A combustion condition detecting device is proposed in U.S. Pat. No. 6,104,195 (JP-A-9-25867). In this device, an AC voltage is applied between opposing electrodes of an spark plug immediately after ignition. Then, a current flowing between the electrodes is measured. A capacitive current component generated by the AC voltage is eliminated from the detected current. Therefore, only a combustion ion current component can be extracted.
However, the output level of the ion current is generally low. Especially in a lean-burn engine and a stratified charge engine, the output level of the ion current is far lower. As a result, determining of abnormal combustion conditions, such as a misfire or a knock, by the ion current is difficult. Therefore, the level of the ion current needs to be raised in order to improve an accuracy in the ion current detection.
The present invention therefore has an objective to provide an ion current detecting device for an internal combustion engine enabling more accurate ion current detection in order to properly determine combustion conditions.
An ion current detecting device for an internal combustion engine of the present invention includes an ignition coil, a pair of opposing electrodes, an AC voltage applying device and a current detecting device. The ignition coil has primary and secondary windings. The opposing electrodes are connected to the secondary winding of the ignition coil installed in the combustion chamber of the internal combustion engine. The AC voltage applying device applies an AC voltage between the opposing electrodes. The current detecting device detects a current flowing between the opposing electrodes.
In this device, a current flows between the opposing electrodes at the same frequency as the AC voltage during combustion when the AC voltage is applied between the electrodes. The current is detected by the current detecting device. More particularly, combustion ions are generated in the combustion chamber immediately after the combustion. The current caused by the combustion ions (ion current) is detected.
Moreover, the frequency of the AC voltage applied is set close to a resonant frequency of the ion current path on the secondary side of the ignition coil. This causes a lager amount of ion current to flow and raise the level of the ion current. As a result, the accuracy in the ion current detection can be improved and the combustion conditions can be properly determined.